Radar level gauging (RLG) to measure the level of a filling material, such as a liquid or a solid like a granulate is an increasingly important method for level gauging in tanks, containers, etc. Many different types of RLG systems are previously known. An example of such a systems is disclosed in U.S. Pat. No. 7,106,247 by the same applicant.
Due to safety restrictions, environmental laws and requirements, etc, there is often a need for several measurements of the surface level of the medium in the container, which measurements are completely separated and functionally independent from each other. For example, in radar level gauging systems for a tanker's load containers, at least one alarm function (e.g. overfill alarm) that is functionally independent of the level measuring system is required.
Functional independence here means that a fault in one system does not render the other system(s) inoperative. Such independence can be achieved by ensuring that there are no common electrical circuits and cabling, i.e. there must not be any galvanic contact between different measuring systems. One way to achieve this is to simply install at least two complete independent level gauges. However, in order to save costs, it may be permissible (by technical and regulatory requirements) to share fixed mechanical constructions, which cannot normally malfunction.
Several such radar level gauging system providing two or more functionally independent channels are previously known. For example, U.S. Pat. No. 6,414,526 assigned to the same assignee discloses a device and a method for measuring the level of the surface of a filling material in a container involving more than one radar channel.
Independent radar level gauges using the same physical antenna but otherwise electrically independent have found use for redundancy at a low cost. The method to connect more than one sensor to one antenna is a very cost effective way to implement e.g. a system with a level sensor and an independent overfill alarm, etc, and has gained wide acceptance among users and authorities.
However, in some applications radar level gauges using antennas to provide free propagating signals are not suitable, and a wave guiding structure is used to guide the waves. Based on propagation mechanism three different types of wave guiding structures are known in the prior art.
Wave guides of the first type are hollow (e.g. a pipe of suitable cross section) and “thick” in the sense that they have a cross section of half a wavelength or more, possibly reduced by a dielectric filling. The electromagnetic fields in such a waveguide always have at least one field component along the direction of propagation. When used in radar level applications, wave guides of this type are referred to as “still pipes”, and must be perforated to get the same level inside as outside.
Wave guiding structures of the second type are transmission lines with two or more conductors, such as a twin line or a coaxial line. Transmission line wave guides have a diameter much smaller than the wavelength of the transmitted waves, and one typical feature is that the electromagnetic fields are transverse or of TEM-type (Transverse Electro-Magnetic fields). For practical level gauging applications using signals below 1 GHz (with wavelength above 300 mm) a transmission line diameter of 3-20 mm is commonly used. A too small diameter will increase resistive losses and may cause problems with material clogging and mechanical strength.
Finally, wave guiding structures of the third type are surface wave guides (SWGs), such as a single line transmission line or tube with or without dielectric coating. A surface wave guide can be very thin as compared to the wavelength (4-8 mm is a common SWG diameter for use below 1 GHz) but they also have field along the propagation direction and also fields well outside of the SWG. In contrast to the transmission line probe it needs more space free as there are fields outside the wire. In case of a single metal wire, a poor electric conductor such as stainless steel is suitable. The single wire probe is very practical and robust to use for level gauging.
Wave guiding structures of the second and third type thus both have a diameter much smaller than the wavelength of the transmitted waves. In radar level gauging applications such waveguides are normally referred to as “probes”, and the detection principle is sometimes referred to as guided wave radar (GWR). The most common type uses short pulses (around 1 ns) without carrier and occupy a frequency range of roughly 0.1-1 GHz.